What is the definition of metaphor?

A.
a comparison of two things, usually using like or as; for example, "He ran like greased lightning"

B.
a comparison of two things that does not use like or as; for example, "Her eyes are blue pools"

C.
a comparison of one thing to all others; for example, "Jupiter is the largest planet"

D.
a comparison of two people; for example, "Helen is older than Petra"